Recently, portable electronic equipment have become smaller and lighter but have increased in capacity. In light of this, a need has developed for batteries having improved performance for use in these portable electronic devices. Such batteries must be produced cost-effectively manner and be reliable and safe. To satisfy this need, lithium secondary batteries have been developed. These lithium batteries are classified as lithium sulfur batteries, which use sulfur-based materials as the positive active material, and lithium ion batteries, which use lithiated transition metal oxides as the positive active material.
The positive active material is a main component of the lithium secondary battery and is major factor affecting the performance and safety of the battery. The positive active material typically comprises a chalcogenide compound, e.g. a metal oxide complex such as LiCoO2, LiMn2O4, LiNiO2, LiNi1-xCoxO2 (0<x<1), and LiMnO2. The positive active material is mixed with a conductive material such as carbon black, a binder, and a solvent to provide a slurry composition. The slurry composition is then coated on a thin metal plate such as aluminum foil to provide a positive electrode for a lithium ion secondary battery.
Among the above positive active materials, manganese-based materials such as LiMn2O4 and LiMnO2 are easier to prepare and less expensive than the other materials. Manganese-based materials are also are environmentally friendly. However, manganese-based materials have relatively low capacity. In contrast, cobalt-based materials such as LiCoO2 and LiCoO2 are relatively expensive, but are have good electrical conductivity and high cell voltage. For this reason, cobalt-based materials are widely used. Nickel-based materials such as LiNiO2 are inexpensive and have high capacity, but are difficult to form into a desired structure and are less stable in the charged state, causing battery safety problems.
While conventional positive active materials mainly comprise Co-based positive active materials, high capacity Ni-based positive active materials have recently been used to prepare high capacity batteries. However, since Ni-based positive active materials are spherical in shape, the electrode plate can only be fabricated with a maximum density of the active mass (a mixture of the positive active material, a binder, and a conductive agent) of just 3.2 g/cc. The electrode plate is generally fabricated by pressing in order to increase the density of the active mass. The active material powders are compressed to provide an electrode plate having a high density of the active mass by sliding. However, as Ni-based positive active materials have low hardness, these powders do not slide and break from the compression and the density of the active mass therefore does not increase. Accordingly, even though the active material itself has a high capacity, the density of the active mass is too low to provide a high capacity battery.